1. Field of the Invention
The present invention generally relates to an apparatus, method and computer program for correcting a common phase error (CPE) of symbols of a received orthogonal frequency division multiplexing (OFDM) signal.
2. Description of the Related Art
In general, a digital High Definition Television (HDTV) broadcast system may be divided into two parts, i.e., an image encoder and a modulator. The image encoder may compress about 1 Gbps of digital data, obtained from a high definition image source, for example, into about 15˜18 Mbps of data. The modulator may transmit several Mbps of digital data to a receiving part of the HDTV broadcast system through a substantially narrow channel having about a 6˜8 MHz frequency band.
A modulation technique used in an HDTV broadcast system typically may need high frequency band efficiency in order to transmit several Mbps of digital data through such a limited frequency band to a receiving part of the system. In addition, since an HDTV broadcast system may adopt a simultaneous terrestrial broadcast manner using VHF/UHF channels originally allotted for a conventional analog television broadcast, the HDTV broadcast system may require given characteristics so that the system may be sufficiently robust against channel interference that may be caused by analog television signals.
In an effort to enhance the efficiency of data transmission per bandwidth, and also to reduce and/or possibly prevent interference, orthogonal frequency division multiplexing (OFDM) has been adopted as a next-generation HDTV terrestrial broadcast technique. OFDM is digital modulation in which a column of symbols input in series is transformed into parallel data symbols, and the parallelized symbols are multiplexed into different subcarrier frequencies. In OFDM, multiple carriers which are orthogonal to one another may be used. Two carrier waves (‘carriers’) may be said to be orthogonal if the dot product of the two carrier waves is equal to zero. The use of orthogonal subcarriers allows the subcarriers' spectra to overlap, thus possibly increasing spectral efficiency. As long as orthogonality is maintained, it may still be possible to recover the individual subcarriers at the receiver with accuracy, despite the overlapping spectrum of the subcarriers.
Since OFDM signals may be generated by multiple carrier frequencies, OFDM signals may be vulnerable to tuner phase noise. Tuner phase noise may affect an OFDM signal in at least two different ways. For example, tuner phase noise may introduce low-frequency phase variations. The result of these variations may be a constant phase rotation for all of the subcarriers in the frequency domain. This phenomenon is referred to as a common phase error (CPE). Second, tuner phase noise may cause inter-carrier interference (ICI), which is determined by high-frequency phase noise variations. The result of the ICI may be signal-to-noise ratio degradation due to interference from the other subcarriers. The occurrence of the two phenomena, i.e., CPE and ICI, may depend on the spectral properties of the phase noise and frequency spacing between adjacent subcarriers of the OFDM signal.
CPE generally may cause HDTV broadcast system performance to deteriorate. However, CPE may be estimated based on the received OFDM signals and then corrected. Conventional CPE correction methods have been suggested by J. H. Scott in the article entitled “The DVB Terrestrial (DVB-T) Specification and Its Implementation to a Practical Modem” (International Broadcasting Convention, 1996) and P. Robertson and S. Kaiser in the article entitled “Analysis of the Effects of the Phase Noise in Orthogonal Frequency Division Multiplexing (OFDM) Systems” (IEEE Int. Conf. On Communication, ICC'95).
FIG. 1 is a block diagram of a prior art OFDM signal receiving apparatus. An OFDM signal; in general, may include a plurality of subcarriers having a given frequency, amplitude, and phase. These subcarriers may be referred to as continual pilots (CPs). For example, a Digital Video Broadcast-Terrestrial (DVB-T) system uses 45 CPs in a 2K mode and uses 177 CPs in a 8K mode. Referring to FIG. 1, in a OFDM signal receiving apparatus 100, an input signal IN may be converted into an intermediate frequency signal by a tuner 102 and a local oscillator 104. The intermediate frequency signal may be converted into a digital signal by an A/D converter 106 and converted into a complex baseband signal by an IQ demodulator 108. The output of the IQ demodulator 108 may be provided to a Fast Fourier Transform (FFT) unit 110 so that a guard interval is removed from each OFDM symbol of the received complex baseband signal (i.e., of the OFDM signal) for carrying out fast Fourier transformation in order to transform time-domain signals into frequency-domain signals.
A first CP extractor 114 may extract first CP signals CP1 of a given symbol of the OFDM signal from an output of the FFT unit 110, and may transmit the CP1 signals to a complex conjugate multiplier 118. The output of the FFT unit 110 may also be transmitted to a second CP extractor 116 via a one-symbol delay unit 112. The second CP extractor 116 may extract second CP signals CP2 from a previous symbol of the OFDM signal passing through the one-symbol delay unit 112, and may transmit the CP2 signals to the complex conjugate multiplier 118. The complex conjugate multiplier 118 may multiply the CP1 and CP2 signals, which may have the same frequencies, thus detecting phase differences between the CP1 and CP2 signals.
An output of the complex conjugate multiplier 118 may be transmitted to an averaging circuit 120. The averaging circuit 120 may average detected phases of all CP signals in a given symbol received from the complex conjugate multiplier 118, in order to remove noise components. The averaging circuit 120 may output a complex signal to a phase extractor 122. The phase extractor 122 may detect the phase of the complex signal. Phase detection outputs of the phase extractor 122 may be accumulated in an accumulator 124. The accumulator 124 may provide an accumulated output to a phase rotator 126, so that the one-symbol-delayed signal, output from the one-symbol delay unit 112, may be rotated in an opposite direction.
CPE correction by the prior art OFDM signal receiving apparatus 100 may be performed as follows. CP subcarriers of a current OFDM symbol may be multiplied by complex conjugates of subcarriers from a previous OFDM symbol, which may be expressed by Equation (1).
                                                        Ri              ⁡                              [                kcp                ]                                      ·            Ri                    -                      1            *                          [              kcp              ]                                      =                                                            ⅇ                                  j                  ⁡                                      (                                                                  ϕ                        i                                            -                                              ϕ                                                  i                          -                          1                                                                                      )                                                              ⁢                                                                                      H                    ⁡                                          [                      kcp                      ]                                                                                        2                            ⁢                              Xi                ⁡                                  [                  kcp                  ]                                            ⁢              Xi                        -                          1              *                              [                kcp                ]                                              =                                    16              9                        ⁢                                                                            H                  ⁡                                      [                    kcp                    ]                                                                              2                        ⁢                          ⅇ                              (                                                      ϕ                    i                                    -                                      ϕ                                          i                      -                      1                                                                      )                                                                        (        1        )            
In Equation (1), Ri[k] may represent a signal received after FFT, i.e., a k-th subcarrier of an i-th OFDM symbol. H[k] may represent a channel frequency response to the k -th subcarrier, kcp may represent the location of a CP, and 16/9 represents the power of a CP subcarrier, as defined by the standards specified for DVB-T systems.
A differential phase error, ΔCPEi,i−1, may then be obtained by averaging one OFDM symbol and by extracting the phase of an averaged signal, expressed by Equation (2).
                              Δ          ⁢                                          ⁢                      CPE                          i              ,                              i                -                1                                                    =                              ∠            ⁡                          [                                                E                  ⁡                                      (                                                                  16                        9                                            ⁢                                                                                                                              H                            ⁡                                                          [                              k                              ]                                                                                                                                2                                            ⁢                                              ⅇ                                                  j                          ⁡                                                      (                                                                                          ϕ                                i                                                            -                                                              ϕ                                                                  i                                  -                                  1                                                                                                                      )                                                                                                                )                                                  ⁢                                  ❘                                      k                    ⋐                    kcp                                                              ]                                =                                    ∠ⅇ                              j                ⁡                                  (                                                            ϕ                      i                                        -                                          ϕ                                              i                        -                        1                                                                              )                                                      =                                          ϕ                i                            -                              ϕ                                  i                  -                  1                                                                                        (        2        )            
A CPE, which may be obtained by accumulating differential phase errors, may be calculated using Equation (3).
                              CPE          i                =                                            ∑                              j                =                1                            i                        ⁢                                                  ⁢                          Δ              ⁢                                                          ⁢                              CPE                                  j                  ,                                      j                    -                    1                                                                                =                                    (                                                ϕ                  i                                -                                  ϕ                                      i                    -                    1                                                              )                        +                          (                                                                    ϕ                                          i                      -                      1                                                        -                                      ϕ                                          i                      -                      2                                                        +                  …                  +                                      (                                                                  ϕ                        i                                            -                                              ϕ                        0                                                              )                                                  =                                                      ϕ                    1                                    -                                      ϕ                    0                                                                                                          (        3        )            
The calculated CPE includes a given level of phase offset φ0. However, since the given phase offset φ0 may be compensated for by the equalizer 128, the given phase offset φ0 may have a minimal adverse affect on HDTV system performance.
FIG. 2 illustrates the structure of an OFDM frame; and FIG. 3 is a diagram illustrating the structure of an OFDM symbol. FIGS. 2 and 3 are provided to illustrate the relatively complicated hardware structure of the OFDM signal receiving apparatus 100 of FIG. 1, due to the inclusion of the one-symbol delay unit 112.
As shown in FIG. 2, OFDM symbols may be classified into 2K mode OFDM symbols and 8K mode OFDM symbols according to the FFT sizes specified by the DVB-T standards. As shown in FIG. 3, CPs included within a given OFDM symbol may have corresponding designated locations within the given symbol. These designated locations may be different from designated locations in another given OFDM symbol. Therefore, as seen from a direction of time, one OFDM symbol may be understood as a continuous signal comprised of data, CPs, and transmission parameter signals, as shown in FIG. 3.
In DVB-T 8K mode, an OFDM symbol includes 6817 subcarriers. Supposing that I and Q data are each represented by 10 bits, the one-symbol delay unit 112 thus requires memory capable of storing as many as 136340 bits (6817×10×2=136340). Accordingly, the hardware structure of the OFDM signal receiving apparatus 100 may be cumbersome and require a relatively complex configuration, which may also require additional space or area to accommodate a larger OFDM signal receiving apparatus 100.